1. Field of the Invention
The present invention relates to a thin-film common mode filter and a thin-film common mode filter array.
2. Description of the Related Art
Common mode filter is a device for suppressing common mode currents that cause electromagnetic interference in parallel transmission lines. The common mode filter has magnetically coupled inductors to remove in-phase noise component.
Thin-film common mode filter miniaturized and highly integrated by forming bilayered thin-film coils between ferrite substrates and by constructing in chip form, and thin film common mode filter array on which a plurality of the filters are mounted, are described in for example, Japanese Patent Publications Nos. 04-364709A, 08-203737A, 08-335517A and 11-054326A.
High-frequency properties of such a thin-film common mode filter are effectively improved by smaller capacities between coil conductors and between coil conductors and lead conductors. However, higher aspect ratios (height/width) in the coil and lead patterns for reducing the capacities cause the tolerance of the optimum exposure conditions for forming contact holes as connection portions between upper coil conductor and upper lead conductor to be unlimitedly lower. As the results, insulating materials in the contact hole portion become difficult to be dissolved during development. Therefore, a reliability in electrical conduction becomes lower, and an insulation failure between the upper coil conductor and the upper lead conductor is likely to occur by a decrease in a film thickness of the insulating material.